In-Situ Photo-Dissociation and Polymerization of Carbon Disulfide with Vacuum Ultraviolet Microplasma Flat Lamp for Organic Thin

applied sciences

Article In-Situ Photo-Dissociation and Polymerization of Carbon Disulﬁde with Vacuum Ultraviolet Microplasma Flat Lamp for Organic Thin Films

Jinhong Kim 1 and Sung-Jin Park 1,2,*

1 Laboratory of Optical Physics and Engineering, Department of Electrical and Computer Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA; [email protected] 2 Eden Park Illumination, Inc., Champaign, IL 61821, USA * Correspondence: [email protected]

Abstract: Vacuum UV (VUV) photo-dissociation for a liquid phase organic compound, carbon

disulﬁde (CS2), has been investigated. 172 nm (7.2 eV) VUV photons from Xe2* excimers in a microcavity plasma lamp irradiated free-standing liquid droplets on Si substrate in each a nitrogen

environment and an atmospheric air environment. Selective and rapid dissociation of CS2 into C-C, C-S or C-O-S based fragments was observed in the different gas environments during the reaction. Thin-layered polymeric microdeposites have been identiﬁed by characterization with a Scanning electron microscope (SEM), Energy dispersive x-ray spectroscopy (EDX), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). This novel photo-process from the ﬂat VUV microplasma lamp introduces another pathway of low-temperature organic (or synthetic) conversion for large area deposition. The in-situ, selective conversion of various organic precursors can be potentially used in optoelectronics and nanotechnology applications.

Citation: Kim, J.; Park, S.-J. In-Situ Keywords: carbon disulfide; microplasma; photo-dissociation; polymerization; 172 nm VUV lamp Photo-Dissociation and Polymerization of Carbon Disulfide with Vacuum Ultraviolet Microplasma Flat Lamp for Organic Thin Films. Appl. Sci. 2021, 11, 2597. 1. Introduction https://doi.org/10.3390/app11062597 Low temperature deposition techniques are becoming essential for many types of opto- and microelectronics fabricated in flexible substrates. Several approaches are currently Academic Editor: Andrea Li Bassi being utilized to avoid damage on fragile substrates, such as plasma-enhanced chemical deposition (PECVD) and laser chemical deposition (LCVD) [1,2]. However, malleable Received: 18 February 2021 substrates are subjected to high energetic ionic bombardment caused by plasma generation Accepted: 12 March 2021 in the PECVD system, producing cracking and unwanted stoichiometry ratios in the Published: 15 March 2021 deposited material [3–5]. In an LCVD system, lasers are costly and provide only a small area deposition about several cm2. Alternatively, an incoherent UV source can be used Publisher’s Note: MDPI stays neutral in many processing applications, since most organic/inorganic materials readily absorb with regard to jurisdictional claims in photons in wavelengths < ~250 nm. However, the utilization of traditional UV sources published maps and institutional affil- (UVC) in chemical dissociation is still limited by insufficient photon energy associated with iations. reactants’ chemical bonds, and ends in slow process rates (kinetics) unless it uses photo- catalysts. A new VUV (172 nm) flat lamp [6] has distinct benefits such as high radiation power, large and uniform area coverage that was not possible in the conventional photon sources for chemical reactions. In this research article, the experiment for dissociation Copyright: © 2021 by the authors. of CS2 by a VUV lamp will be described in the different experimental conditions. As a Licensee MDPI, Basel, Switzerland. conductive polymer, CS2 has been used as a sulfur dopant in organic chemistry and a This article is an open access article solvent for various polymers [7,8]. In addition, CS2 can be used as an etchant of the carbon distributed under the terms and layer when it is used with plasma [9]. It has been known that CS2 can be readily dissociated conditions of the Creative Commons to CS + S fragments through photons, catalytic reactions, and plasma processes in the Attribution (CC BY) license (https:// vapor phases [10–12]. However, direct photodissociation of a free standing and volatile creativecommons.org/licenses/by/ CS droplets on a substrate using high energy VUV photons (>7.2 eV) to deposit a thin 4.0/). 2

Appl. Sci. 2021, 11, 2597. https://doi.org/10.3390/app11062597 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, x FOR PEER REVIEW 2 of 9

in the vapor phases [10–12]. However, direct photodissociation of a free standing and vol-

Appl. Sci. 2021, 11, 2597 atile CS2 droplets on a substrate using high energy VUV photons (>7.2 eV)2 to of 10deposit a thin layer that has never been explored. In addition, the conversion of the photo-fragments to a thin layer deposit in a scaled area of Si substrate is introduced. Being the first report of its kind for an in-situ high energy UV deposition methodology, the low temperature layer that has never been explored. In addition, the conversion of the photo-fragments to anda thin simple layer depositdeposition in a scaledtechnology area of of Si organic substrate precursors is introduced. will Being lead theto the ﬁrst cost-effectiveness report of ofits microelectronics kind for an in-situ processes. high energy UV deposition methodology, the low temperature and simple deposition technology of organic precursors will lead to the cost-effectiveness of 2.microelectronics Experimental processes. Method

2. ExperimentalThe operation Method principle of excimer lamps relies on the radiative decomposition of excimer dimers such as Xe2* (172 nm) formed in a plasma discharge of noble gases. As The operation principle of excimer lamps relies on the radiative decomposition of illustrated in Figure 1a, Xe2 is well known for emitting bright continua (150, 172 nm) along excimer dimers such as Xe2* (172 nm) formed in a plasma discharge of noble gases. As with resonance lines of Xe* (129 and 147 nm). The dominant Xe2 emission at 172 nm arises illustrated in Figure1a, Xe 2 is well known for emitting bright continua (150, 172 nm) along 1 1 fromwithresonance A ∑ → linesX ∑ of transitions Xe* (129 and of 147 the nm). molecule The dominant and its Xe emission2 emission intensities’ at 172 nm arises growth are 1 + 1 + attributedfrom A ∑u to→ theX ∑ three-bodyg transitions collision of the molecule of excited and state its emission Xe* and intensities’ 2Xe ground growth state are atoms [6]. Theattributed formation to the rate three-body of these collision three ofbody excited reactions state Xe* is andincreased 2Xe ground quadratically state atoms with [6]. Xe gas pressure.The formation The ratestructure of these of threethe microplasma body reactions lamp is increased used here quadratically has an array with of Xe microcavities gas thatpressure. produces The structure a low temperature of the microplasma and non-equi lamp usedlibrium here plasmas. has an array The of electron microcavities temperature that produces a low temperature and non-equilibrium plasmas. The electron temperature of microplasma lies in the 2–5 eV interval, which is ideal for forming the excimer (Xe2) and of microplasma lies in the 2–5 eV interval, which is ideal for forming the excimer (Xe2) increasingand increasing its transition its transition intensities. intensities. Another Another advantageadvantage of of using using excimer excimer lamps lamps is that is that the non-stablethe non-stable ground ground state state of ofan an excimer excimer lamp lamp is is able able to have nono self-absorptionself-absorption of of the the emit- tedemitted illumination, illumination, providing providing a ahigh high UV UV intensity. intensity. InInaddition, addition, the the ﬂat flat form form factor factor and and dif- fusivediffusive glow glow in in the the lamp lamp makemake a a uniform uniform treatment treatment over over thearea the ofarea the of lamp. the Therefore,lamp. Therefore, thethe depositiondeposition area area can can be be easily easily scaled scaled along along with with the ﬂat the lamp flat sizelamp at size given at intensities. given intensities.

Figure 1.1.( a(a)) Basic Basic energy energy and and transition transition diagram diagram of VUV of emission VUV emission of Xe2 excimer of Xe2 lamp, excimer (b) Schematic lamp, (b) Sche- maticpotential potential energy curveenergy of thecurve target of molecule,the target CS molecule,2 with 172 CS nm2 VUVwith lamp 172 nm energy VUV level lamp (red energy dash line). level (red dash line). Appl. Sci. 2021, 11, 2597 3 of 10

Appl. Sci. 2021, 11, x FOR PEER REVIEW 3 of 9 Figure2a demonstrates the operation of the 172 nm microplasma VUV lamp (100 × 100 mm, Eden Park Illumination, Inc. Champaign, IL, USA). The lamp is filled with 500 Torr of 70% Xe/Ne gas mixture, and operated at 20–30 kHz of 3 kV pulses. The outputFigure intensity 2a demonstrates at the surface the operation of the lamp of the was 172 17nm mW/cm microplasma2. Figure VUV2 lampb shows (100 the× VUV emission100 mm, Eden spectrum Park Illumination from the lamp., Inc. FigureChampaign,1b is aIL, schematic USA). The of lamp the potentialis filled with energy 500 curves Torr of 70% Xe/Ne gas mixture, and operated at 20–30 kHz of 3 kV pulses. The output in the excited states of CS [13]. The red dashed line corresponds to the 172 nm VUV lamp intensity at the surface of the2 lamp was 17 mW/cm2. Figure 2b shows the VUV emission photon energy (7.2 eV) in this experiment. Photon energy from the VUV lamp sufficiently spectrum from the lamp. Figure 1b is a schematic of the potential energy curves in the covers the energies required to dissociate the C=S bond of the CS . The configuration of excited states of CS2 [13]. The red dashed line corresponds to the 172 nm 2VUV lamp pho- theton VUVenergy photodissociation (7.2 eV) in this experiment. setup were Phot shownon energy in Figure from2 c.the The VUV distance lamp sufficiently between the VUV lampcovers and the substrateenergies required was maintained to dissociate 1 the cm. C=S VUV bond photons of the CS are2. The affected configuration by environmental of conditionsthe VUV photodissociation as it is readily setup absorbed were shown by surrounded in Figure 2c. chemical The distance species between such the as VUV water vapor orlamp oxygens. and substrate It means was that maintained VUV photons 1 cm. VU cannotV photons travel are long affected distances. by environmental The experiment has beenconditions performed as it is in readily the N absorbed2 (purged) by environment, surrounded chemical which doesspecies not such absorb as water the 7.2 vapor eV photons, toor minimizeoxygens. It the means photon that lossesVUV photons (and observe cannot thetravel contribution long distances. of VUV The experiment photons to has the overall reaction).been performed Besides, in the we N also2 (purged) performed environment, the experiments which does at not ambient absorb (drythe 7.2 air) eV environmentpho- preferringtons, to minimize the formation the photon of losses active (and oxygen observe species the contribution by moisture of VUV and photons oxygen. to the The latter conditionoverall reaction). led to Besides, the deposition we also performe of a seriesd the of experiments compounds at (associated ambient (dry with air) oxidation,envi- as ronment preferring the formation of active oxygen species by moisture and oxygen. The shown in results from EDS). The distance of 1 cm was chosen from the multiple tests to latter condition led to the deposition of a series of compounds (associated with oxidation, findas shown the optimalin results distance, from EDS). which The distance maintains of 1 cm the was consistency chosen from between the multiple two tests experimental to approachesfind the optimal and distance, keeping which a balance maintains between the consistency photochemistry between and two direct experimental photo-radiation. ap- A shorterproaches distance and keeping than 1a cmbalance was notbetween considered photochemistry in these experimentsand direct photo-radiation. to avoid any interactionA betweenshorter distance the target than substrates 1 cm was not (or considered chemical vapors)in these experiments and the flat to lamp avoid itself. any interac- Free-standing, liquidtion between phase the CS target2 droplets substrates (99.99%, (or chemical Sigma-Aldrich, vapors) and St. the Louis, flat lamp MO, itself. USA) Free-stand- were applied to theing, Si liquid substrate phase withCS2 droplets a micropipette (99.99%, Sigma-Aldrich, for quantitative St. Louis, sampling MO, fromUSA) 0.2were mL applied to 3 mL. The chamberto the Si substrate was purged with a withmicropipette an atmospheric for quantitative pressure sampling of either from 0.2 research-grade mL to 3 mL. The N 2 or dry airchamber with awas flow purged rate with of 5 an slm. atmospheric After VUV pressure irradiation of either to research-grade the target substrate, N2 or dry theair treated sampleswith a flow were rate analyzedof 5 slm. After by SEM VUV (4700irradiation Hitachi), to the EDXtarget (iXRF substrate, Oxford the treated Instruments), samples Raman spectroscopywere analyzed (Ramanby SEM (4700 11, Nanophoton), Hitachi), EDX and(iXRF XPS Oxford (Axis Instruments), ULTRA, Kratos) Raman to spectros- investigate the copy (Raman 11, Nanophoton), and XPS (Axis ULTRA, Kratos) to investigate the chemical chemical composition and morphology of the deposited films. composition and morphology of the deposited films.

Figure 2. (a) Photographs of microplasma VUV lamp operation and an expanded view of a

lamp surface, (b) VUV emission spectrum of the lamp, (c) Schematic of CS2 synthesis from VUV lamp illumination. Appl. Sci. 2021, 11, x FOR PEER REVIEW 4 of 9

Appl. Sci. 2021, 11, 2597 Figure 2. (a) Photographs of microplasma VUV lamp operation and an expanded view of a lamp4 of 10 surface, (b) VUV emission spectrum of the lamp, (c) Schematic of CS2 synthesis from VUV lamp illumination.

3.3. DiscussionDiscussion

TheThe photo-dissociation of of free-standing free-standing CS CS2 liquid2 liquid samples samples were were performed performed as a func- as a functiontion of time of time and anddifferent different sample sample volumes. volumes. The Theimmediate immediate photo-dissociation photo-dissociation and a and de- aposit deposit formation formation were were observed observed within within one oneminute minute of VUV of VUV irradiation. irradiation. After After 3 min, 3 min,most mostliquid liquid phase phase samples samples disappear disappear due to due photo-dissociation to photo-dissociation or vaporization or vaporization of the original of the originalcompound. compound. Figure 3 Figureshows3 the shows SEM the images SEM of images microdeposits of microdeposits formed on formed the Si on substrate the Si substrateafter 3 min after of VUV 3 min irradiation of VUV irradiation in various inCS various2 sample CS volumes.2 sample The volumes. deposited The area deposited in each areasample in each is proportional sample is proportional to the quantitative to the quantitative volumes of volumesCS2. The ofdarker CS2. Thearea darkeris a deposited area is aarea deposited from the area CS from2 precursor the CS2 decomposition,precursor decomposition, and lighter and areas lighter indicate areas the indicate Si substrate. the Si substrate.When the WhenCS2 precursor the CS2 precursor is applied is to applied the Si substrate, to the Si substrate, it immediately it immediately evaporates evaporates without withoutleaving any leaving residues. any residues. However, However, the 172 nm the ph 172otons nm convert photons the convert free standing the free precursors standing precursorsinto different into morphology different morphology films in either ﬁlms N in2 eitheror air Nenvironment,2 or air environment, coveringcovering the entire the Si entiresubstrate. Si substrate.

FigureFigure 3.3. SEMSEM imagesimages ofof microdeposits formed on a Si substrate substrate during during the the irradiation irradiation of of 172 172 nm nm VUVVUV photonsphotons forfor 33 min.min. TheThe differentdifferent amountsamounts ofof precursorsprecursors are irradiated: ( a) 0.2 mL, ( (b)) 0.5 0.5 mL, mL, (c) 1 mL, (d) 3 mL of CS2 under nitrogen environment at 300 K. (c) 1 mL, (d) 3 mL of CS2 under nitrogen environment at 300 K.

TheThe VUVVUV photons also make make a a selective selective reaction reaction pathway pathway to toa specific a specific product product by bythe themodification modification of reaction of reaction para parametersmeters during during the the process. process. The The initial initial process process condition condition of ofan an N2 N environment2 environment was was chosen chosen to form to form an inert an inert condition condition because because N2 does N2 does not react not react with withVUV. VUV. It also It alsohelps helps VUV VUV photons photons travel travel to the to target the target substrate substrate without without any absorption any absorption from fromenvironmental environmental contaminants. contaminants. On the On other the hand, other the hand, ambient the ambient O2 gas Oefficiently2 gas efficiently absorbs absorbsthe VUV the photons VUV photons to form toan form oxyg anen oxygenatom species atom species(O) or ozone (O) or (O ozone3). These (O3). new These species new speciescan modify can modify the existing the existing reaction reaction mechanism mechanism of the ofCS the2 dissociation CS2 dissociation process process and create and createalternative alternative synthetic synthetic pathways. pathways. The experiment The experimentss under the under air environment the air environment were made were to madecompare to comparethe photochemistry the photochemistry of N2. Samples of N 2from. Samples air environm from airent environment yielded a different yielded mor- a differentphology morphologythan was obtained than wasfrom obtained the N2 environment. from the N2 Theenvironment. formation Therate formationin air was close rate into airthe was rate closein the to N the2 environment. rate in the NFigure2 environment. 4 shows the Figure results4 showsof EDX theanalysis results for of deposit EDX analysis for deposit samples on the Si substrate, and the table provides the information of a possible composition of identified components in the deposited layer. For the test samples obtained in the N2 environment (Figure4a), the atomic percentages of carbon (C) and sulfur (S) exhibit 15.7% and 0.35%, respectively. Only a negligible content of S element was observed in the layer. It strongly indicates that high energy photons of the microplasma Appl. Sci. 2021, 11, x FOR PEER REVIEW 5 of 9

samples on the Si substrate, and the table provides the information of a possible composition of identified components in the deposited layer. For the test samples obtained in the N2 environment (Figure 4a), the atomic percentages of carbon (C) and sulfur (S) exhibit 15.7% and 0.35%, respectively. Only a negligible content of S element was observed in the Appl. Sci. 2021, 11, 2597 5 of 10 layer. It strongly indicates that high energy photons of the microplasma lamp effectively dissociates the C=S bond of CS2 and creates a deposit with carbon-rich composites (with a possible base-bonding unit of C-C and/or C-S-C). By switching the gas environment from lamp effectively dissociates the C=S bond of CS2 and creates a deposit with carbon-rich N2 to air in Figure 4b,composites the atomic (with percentages a possible base-bonding of C, S, and unit O were of C-C 20.37%, and/or 1.49%, C-S-C). Byand switching 1.5%, the 2 respectively. As is discussedgas environment previously, from N2 strongto air in co-absorption Figure4b, the atomic of O percentages for VUV ofphotons C, S, and re- O were sults in the participation20.37%, of 1.49%, oxygen and 1.5%,species respectively. in the chemical As is discussed reactions. previously, The strongmost co-absorptionprobable of product from this oxidativeO2 for VUV process photons is results the fo inrmation the participation of O-C-S-based of oxygen species compounds, in the chemical and it reactions. is The most probable product from this oxidative process is the formation of O-C-S-based a standard component identified in other oxidation processes of CS2 [14,15]. It should be compounds, and it is a standard component identiﬁed in other oxidation processes of also noted that the atomic percentage ratio of each elements is considered as a relative CS2 [14,15]. It should be also noted that the atomic percentage ratio of each elements proportional valuesis because considered of asthe a relativeinvolvemen proportionalt of Si valueswafer because(substrate) of the as involvement a primary of ele- Si wafer mentsl in the EDS analysis.(substrate) as a primary elementsl in the EDS analysis.

FigureFigure 4. 4.EDS EDS analysis analysis for thefor depositedthe deposited layers. layers. The numerical The numerical analysis resultsanalysis are results listed at are tables: listed (a )at VUV tables: photodissociation(a) VUV photo-dissociation in N2,(b) in atmospheric in N air.2, The(b) in white atmospheric square marks air. (in The the leftwhite hand square side images) marks in (in the the SEM left microscope hand imagesside images) indicate the in analysisthe SEM area microscope in the sample. images indicate the analysis area in the sample. Figure5 shows the Raman spectroscopy for the deposit compounds as well as the Figure 5 showsspectrum the Raman from thespectroscopy Si substrate as fo ar reference. the deposit CS2 is compounds a strong Raman as scatter well due as tothe its sym- spectrum from the metricSi substrate chemical as structure a reference. and vibration CS2 modes.is a strong The comparison Raman scatter of vibrational due modesto its in the symmetric chemicalRaman structure spectra and between vibration the CS modes.2 driven productsThe comparison is straightforward of vibrational[16,17]. In modes the spectra of −1 two deposit samples, any vibrations of CS2 molecule (typically at 656 and 800 cm ) were in the Raman spectra between the CS2 driven products is straightforward [16,17]. In the identiﬁed, indicating that the CS2 was fully decomposed by VUV irradiation. Each deposit 2 spectra of two depositformed samples, at N2 or any air environment vibrations does of CS not showmolecule any similarities (typically in theat spectra.656 and Both 800 Raman − cm−1) were identified,spectra indicating in Figure that5a,b showthe CS a broad2 was baseline fully decomposed band in the range by ofVUV1800–3500 irradiation. cm 1, which Each deposit formedwas at notN2 foundor air inenvironment either CS2 or Sidoes substrate, not show and shows any similarities the characteristic in the of aspec- polymeric tra. Both Raman spectralayer orin multi-elementFigure 5a,b show composites. a broad Among baseline them, band the continuum in the range peak of in the1800– range of 2300 cm−1 to 2700 cm−1 and another peak at 2883 cm−1 are related to the carbon related −1, 3500 cm which wasvibrations not found and in CH either stretching CS2 or vibration Si substrate, of the deposited and shows ﬁlms, the respectively characteristic [18–21 ]. In of a polymeric layerthese or multi-element cases, the existence composites. of such a broad Among peak inthem, the Raman the continuum spectra could peak be assumably in the range of 2300 cmattributed−1 to 2700 to cm a morphology−1 and another of the polymenricpeak at 2883 layer cm or− electronic1 are related Raman to scatteringthe carbon [22]. The −1 related vibrations andpeak CH at 3390 stretching cm indicate vibration the synthesis of the of thedeposited amorphous films, status respectively of carbon. The broad[18– ones at 1550 cm−1 correspond to C-O vibrations [23]. The sharp transition at ~2336 cm−1 arises 21]. In these cases, the existence of such a broad peak in the Raman spectra could be as- from ambient nitrogen (molecular) gas (Q-branch) [24,25] and the peak at 1586 cm−1 is sumably attributed to a morphology of the polymenric layer or electronic Raman scattering [22]. The peak at 3390 cm−1 indicate the synthesis of the amorphous status of carbon. The broad ones at 1550 cm−1 correspond to C-O vibrations [23]. The sharp transition at ~2336 cm−1 arises from ambient nitrogen (molecular) gas (Q-branch) [24,25] and the peak Appl. Sci. 2021, 11, x FOR PEER REVIEW 6 of 9

Appl. Sci. 2021, 11, 2597 6 of 10

at 1586 cm−1 is assigned as ambient O2 scattering. Both peaks are found due to the atomospheric scattering by the collimated probing laser beam at the substrate, as notated in Fig- assigned as ambient O2 scattering. Both peaks are found due to the atomospheric scattering ureby the 5c collimated [26]. probing laser beam at the substrate, as notated in Figure5c [26].

Figure 5. Raman spectrum analysis of the micro-deposit on a Si substrate formed by VUV illumination

Figureunder (a)N 5.2 environmentRaman spectrum (b) atmospheric analysis air, (c) Si substrate of the (formicro-deposit comparison), respectively. on a Si substrate formed by VUV illumination under (a) N2 environment (b) atmospheric air, (c) Si substrate (for comparison), respec- The chemical bondings formed in the deposited films after VUV irradiation were tively.investigated by XPS. Representative C 1s and S 2p XPS spectra of a deposited film including deconvoluated peak assignments are illustrated in Figure6. Figure6a,b show the C 1s and S 2p core level spectra of the sample after VUV irradiation with air environment, respectively. The deconvoluationThe chemical of C 1s bondings spectrum is composedformed of in two the curves depo whichsited correspond films toafter VUV irradiation were in- vestigatedthe carbon bonding by XPS. (sp3) and Representative C-O bonding [27–30 ].C S 1s 2p peakand can S 2p be deconvoluted XPS spectra into of a deposited film including three species, corresponding to the C-S and S-O bond [30–32]. As shown in Figure6c, the deconvoluated288 eV component is tentatively peak assignments assigned to amide are C (N-C=O), illustrated carboxyl in C Figure (O-C=O) and6. Figure 6a,b show the C 1s and Scarbonyl 2p core C (C=O) level [30 ,33spectra]. While XPSof measurementsthe sample of after samples VUV with VUV irradiation irradiation in with air environment, respec- N2 environment shows completely different spectra in Figure6c,d. The spectrum of sp3 C tively.shows relatively The deconvoluation higher intensity than C-O of bondC 1s in spectrum Figure6c which is wascomposed the opposite of to two curves which correspond tothe the spectrum carbon shown bonding in Figure6a. (sp3) This is because and C-O larger bonding fluence of VUV [2 photons7–30]. fromS 2p the peak can be deconvoluted into microplasma lamp reached to CS2 precursor in N2 environment, resulting in a higher C-C threebond than species, C-O bond corresponding spectra. However, the to high the absorption C-S and of VUV S-O photons bond by [30–32]. oxygen As shown in Figure 6c, the 288in the eV air environmentcomponent will preventis tentatively delivering photonsassigned to the to substrate amide than C in (N-C=O), the N2 carboxyl C (O-C=O) and environment. Therefore, we expect that O3 or active oxygen species will involve more than carbonylVUV photons C in this(C=O) specific [30,33]. process, which While resulted XPS in ameasurem higher C-O bondents than C-Cof samples bond with VUV irradiation in N2 environment shows completely different spectra in Figure 6c,d. The spectrum of sp3 C shows relatively higher intensity than C-O bond in Figure 6c which was the opposite to the spectrum shown in Figure 6a. This is because larger fluence of VUV photons from the microplasma lamp reached to CS2 precursor in N2 environment, resulting in a higher C-C bond than C-O bond spectra. However, the high absorption of VUV photons by oxygen in the air environment will prevent delivering photons to the substrate than in the N2 environment. Therefore, we expect that O3 or active oxygen species will involve more than VUV photons in this specific process, which resulted in a higher C-O bond than C-C bond spectra in Figure 6a. Besides, the intensity of C-S peaks in air (Figure 6b) was higher than that of Figure 6d by the factor of 10. It shows that the VUV photodissociation of C=S (in CS2) leads the reaction pathways preferably toward C-C polymerization after the complete dissociation of C=S double bonds rather than forming C-S bond based polymeric bonds. The C-S-O bond formation is related to the partial oxidation with the C-S fragments, mainly obtained by direct photodissociation of a C=S bond of CS2 [10]. In the air environment (Figure 6a,b), the formation of the C-O component is preferred over than C- S-O formation due to the oxidative dissociation process of C=S bonds as described above. Appl. Sci. 2021, 11, 2597 7 of 10

spectra in Figure6a. Besides, the intensity of C-S peaks in air (Figure6b) was higher than that of Figure6d by the factor of 10. It shows that the VUV photodissociation of C=S (in Appl. Sci. 2021, 11, x FOR PEER REVIEW 7 of 9 CS2) leads the reaction pathways preferably toward C-C polymerization after the complete dissociation of C=S double bonds rather than forming C-S bond based polymeric bonds. The C-S-O bond formation is related to the partial oxidation with the C-S fragments, mainly obtained by direct photodissociation of a C=S bond of CS2 [10]. In the air environment The results indicate(Figure the 6stronga,b), the possibility formation of the of C-Odesigned component manipulation is preferred over of than different C-S-O formation reaction (deposition) pathwaysdue to through the oxidative the dissociationdifferent processcontrolled of C=S condit bondsions as described (such above.as environment, The results indicate the strong possibility of designed manipulation of different reaction (deposition) photon fluence, or pathwaystime). In through particular, the different all the controlled parameters conditions in (such this as low-temperature environment, photon ﬂuence, photochemistry can be preciselyor time). In (and particular, simply) all the contro parameterslled incompared this low-temperature to the conventional photochemistry reaction can be parameters. precisely (and simply) controlled compared to the conventional reaction parameters.

FigureFigure 6. XPS6. XPS spectra spectra of the of C 1s the and C S 1s 2p and chemical S 2p states chemical in the CS states2 VUV in assisted the CS dissociation2 VUV assisted ﬁlm with dissociation air (a,b) and film nitrogenwith air (c, d()a environment,b) and nitrogen at 300 K ( onc,d Si) substrate. environment at 300 K on Si substrate. 4. Conclusions

4. Conclusions This study describes in-situ, low temperature, photo-assisted dissociation of CS2 on a Si substrate with a VUV (172 nm) microplasma lamp with a ﬂat form factor. Xe2 ex- 2 This study describescimer VUV in-situ, sources low deliver temperat sufﬁcienture, photon photo-assisted energy of 7.2 eV dissociation to the organic of reactant CS on to a Si substrate with a dissociateVUV (172 its C=Snm) bonding microplasma at a higher lamp reaction with rate a within flat form a few minutes.factor. TheXe2 analysisexcimer VUV sources deliver sufficient photon energy of 7.2 eV to the organic reactant to dissociate its C=S bonding at a higher reaction rate within a few minutes. The analysis of the deposited product indicates the possibility of the formation of either a C-C or C-S based polymeric deposit on the substrate. Especially, the investigation of the thin layers by Ra- man spectroscopy and X-ray photoelectron scpectroscpy indicates that the resulting carbon bond was initiated from the CS2 dissociaiton, and it coincides with other reaction mechanisms of the conventional plasma enhanced or photochemical vapor deposition. We have also found that chemical bond and composition could be changed depending on the efficiency of VUV photon delivery, mainly dependent on the controlled environment. It has been known that the material backbones are closely related to the electrical properties of either semiconductors or conductors, depending on its thin-film structure. Amor- phous deposits were also identified in the characterization, and selective formation of the different products was also observed in different environmental parameters associated with VUV absorption. It has the potential to be an efficient technique for in-situ deposition thanks to its simple controllability of deposition parameters and scalable deposition area. Furthermore, the use of other VUV wavelengths from the microplasma lamps (especially for having its photo-energy higher than 7.2 eV) in the microplasma lamp will open a novel Appl. Sci. 2021, 11, 2597 8 of 10

of the deposited product indicates the possibility of the formation of either a C-C or C-S based polymeric deposit on the substrate. Especially, the investigation of the thin layers by Raman spectroscopy and X-ray photoelectron scpectroscpy indicates that the resulting carbon bond was initiated from the CS2 dissociaiton, and it coincides with other reaction mechanisms of the conventional plasma enhanced or photochemical vapor deposition. We have also found that chemical bond and composition could be changed depending on the efficiency of VUV photon delivery, mainly dependent on the controlled environment. It has been known that the material backbones are closely related to the electrical properties of either semiconductors or conductors, depending on its thin-film structure. Amorphous deposits were also identified in the characterization, and selective formation of the different products was also observed in different environmental parameters associated with VUV absorption. It has the potential to be an efficient technique for in-situ deposition thanks to its simple controllability of deposition parameters and scalable deposition area. Furthermore, the use of other VUV wavelengths from the microplasma lamps (especially for having its photo-energy higher than 7.2 eV) in the microplasma lamp will open a novel photochemical tool for the conversion of various organic/inorganic precursors. At present, the 7.2 eV photons efficiently dissociate (initiate the photochemistry) precursor materials containing any chemical double bond structures according to the preliminary studies for the various materials. It is expected that higher photon energies (>8.7 eV) would achieve the photodissociation of, for example, carbon triple bonds in the controlled environment. The VUV-assisted dissociation mechanism at 300 K appears to be very attractive for indus- trial applications in electronic and optical device manufacturing. It is also applicable for environmental applications such as the remediation of harmful organic compounds in the air [15,34].

Author Contributions: Conceptualization, J.K. and S.-J.P.; methodology, J.K. and S.-J.P.; validation, J.K. and S.-J.P.; investigation, J.K. and S.-J.P.; data curation, J.K. and S.-J.P.; writing—original draft preparation, J.K; writing—review and editing, S.-J.P.; supervision, S.-J.P.; project administration, S.-J.P.; funding acquisition, S.-J.P. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Ministry of Environment, Republic of Korea (MOE). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Acknowledgments: This research was supported by R&D Center for Green Patrol Technologies through the R&D for Global Top Environmental Technologies funded by the Ministry of Environment, Republic of Korea (MOE). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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